Add x86_64-linux-gnu platform to Gemfile.lock

Nokogiri 1.18.10 uses x86_64-linux-gnu as the platform identifier
instead of x86_64-linux. The CI workflow runs 'bundle lock --add-platform x86_64-linux'
which was causing the lockfile to be modified because the platform
wasn't pre-resolved.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

Gemfile.lock

@@ -257,6 +257,7 @@ PLATFORMS
   arm64-darwin-24
   x86_64-darwin-20
   x86_64-linux
+  x86_64-linux-gnu
 
 DEPENDENCIES
   appraisal